Hydromechanically regulated vehicle brake system

ABSTRACT

An improved hydraulic brake system accommodates multiple, e.g. primary and secondary actuation forces while maintaining independence of operation between primary and secondary brake subsystems. A secondary subsystem line pressure booster draws its charging fluid from the primary system line using a split piston, free backflow, volume displacement approach. The improved brake system has a temperature sensitive condition sensing hydromechanical fuze to sense hydraulic line flow for velocity, volume, direction and pressure. The brake system also employs an input flow control valve to direct fluid flow in the secondary subsystem.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to brake systems, and moreparticularly to an improved vehicle brake system for use with vehicleswhich employ more than one independent braking subsystem, e.g. whichemploy both hand and foot manual input forces.

2. Description of the Prior Art

It is desirable to have a hydromechanically regulated vehicle brakesystem which accommodates dual hand and foot manual input forces, suchthat deterioration or failure of one of the two subsystems (such as handand foot) will not disable the other. Hydromechanical brake systems arecommonly utilized on recreational vehicles such as all terrain vehicles(ATV) as well as motorcycles. Hydraulic brakes often are used as footbrakes on automobiles. Automotive brake systems use liquid pressure toforce brake pads against rotors on selected wheels and thus stop thewheels from turning. The liquid is contained in actuation devices calledbrake calipers. These calipers are connected by special tubing to apressure and flow generating device called a master cylinder, which isconnected to the hand or foot operated force input lever. When thedriver actuates the lever, the lever pushes a piston in the mastercylinder. This piston forces fluid through the tubes and against thepistons in the calipers. These pistons force the brake pads against thebrake rotors or drums, which slows or stops the wheel from turning.

Many of the basic principles of automotive hydraulic brake operation arecommon to recreational vehicle brake systems which employ hydraulicbraking. Recreational vehicles also employ more than a singleindependent braking system. Examples include a hand operated brake aswell as a foot operated brake. It is desirable to have a system for suchvehicles, which will allow for integrated operation of all independentbraking system calipers from a selected primary system input devicewhich may be hand or foot operated, as well as a secondary mode ofoperation from a second input device.

SUMMARY OF THE INVENTION

The present invention is directed to an improved vehicle brake systemfor use with vehicles which employ more than one independent brakingsubsystem. The novel system employs a hydromechanical sensing device(fuze) to sense hydraulic line flow velocity, volume, pressure andtemperature. A unique feature of this fuze is its ability to set itselfwhen normal system operation parameters are exceeded and then unsetitself if system integrity is sensed. This feature is accomplished viasensing a brake system feedback pressure from a sealed off portion ofthe brake system. The fuze is temperature sensitive and accommodateshydraulic fluid viscosity changes due to temperature variations viaselecting materials having specific thermal coefficients of expansion.The selected materials precisely limit dimensional changes in thehydraulic flow clearances, resulting in temperature compensated flowrates through the fuze assembly.

The novel system also employs a second hydromechanical sensing device(isolator) to seal off and direct hydraulic fluid flow in a secondarysystem. The isolator ensures fully independent operation of eithersubsystem independently. The isolator also allows simultaneous operationof both subsystems. For example, if the primary brake system is damagedor becomes inoperative, the isolator automatically reacts to theresultant lower system pressure and seals off the damaged or unusableportion of the brake system from the remaining subsystem to ensurecontinued operation of a portion of the vehicle brake system.

The novel system also employs a secondary subsystem line pressurebooster. This pressure generating device is similar to a mastercylinder, such as discussed herein above, but unlike a typical mastercylinder, it must draw its charging fluid from the primary system line,as it does not have a vented fluid reservoir. The pressure booster has asplit piston which is directly actuated by manual operator inputs andoperates via free backflow and volume displacement principles.

The present invention provides a solution to the problems andshortcomings inherent in prior art dual hydraulic braking systems anddevices by providing a hydromechanically regulated vehicle brake systemcapable of replacing any vehicle brake system which accommodatesmultiple manual input forces, but which must also maintain independenceof operation between the subsystems such that failure of one of thesubsystems will not disable or impair the integrity of the other.

A feature afforded by the present invention is the provision of a brakesystem which automatically senses all manual operator inputs andautomatically regulates the entire vehicle brake system functions inresponse thereto.

Another feature afforded by the present invention is the provision of abrake system which can easily and swiftly be adapted to retrofitexisting single master cylinder operated brake systems (primary systems)to allow for single point operation of all calipers while simultaneouslyproviding the ability to override subsystems independently of primarysystem operation.

Another feature afforded by the present invention is the provision of abrake system which is completely self-regulating and self-adjusting, andwhich eliminates the necessity for making system or device adjustmentsdue to wear, operating condition fluctuations or changes, or even due toreplacement of worn brake pads.

Yet another feature afforded by the present invention is the provisionof a brake system having primary and secondary hydraulic subsystemswhich share the same fluid circuits and a single input operation, andyet which provides for operator selection to operate the system and oneor more subsystems independently.

Still another feature afforded by the present invention is the provisionof a brake system wherein a primary subsystem normally operates allvehicle brake calipers and wherein a secondary subsystem can be usedindependently or in cooperation with the primary subsystem sharing thesame hydraulic circuits.

Still another feature afforded by the present invention is the provisionof interrelated hydromechanical sensing, regulating and pressureboosting devices in a multiple manual input force operated vehicle brakesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention and many of the attendantadvantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the detailed descriptionwhen considered in connection with the accompanying drawings in whichlike reference numerals designate like parts throughout the figuresthereof and wherein:

FIG. 1 is a simplified block diagram illustrating use of a fuze, flowcontrol valve and secondary subsystem pressure booster to form ahydromechanically regulated vehicle brake system in accordance with oneembodiment of the present invention;

FIG. 2A is a detailed cutaway view of a condition sensinghydromechanical fuze suitable for use in accordance with one embodimentof the present invention as shown in FIG. 1;

FIG. 2B is an enlarged cutaway view more clearly depicting details ofthe retainer portion of the condition sensing hydromechanical fuzeillustrated in FIG. 2A;

FIG. 3A is a detailed cutaway view illustrating an in-line secondarymaster cylinder suitable for use in accordance with one embodiment ofthe present invention as shown in FIG. 1;

FIG. 3B is an enlarged cutaway view more clearly depicting structuraldetails of selected portions of the in-line master cylinder illustratedin FIG. 3A;

FIG. 4 is a detailed cutaway view illustrating a subsystem hydraulicinput flow control valve assembly suitable for use in accordance withone embodiment of the present invention as shown in FIG. 1;

FIG. 5 is a graph illustrating the interactive effects of differentmaterials to precisely control hydromechanical fuze reset times inaccordance with the present invention; and

FIG. 6 is a graph illustrating the efficiency of the input flow controlvalve assembly depicted in FIG. 4 based upon actual valve assembly flowarea as a percentage of maximum practical fluid transmission line flowarea for a standard 3/16-inch fluid line.

While the above-identified drawing figures set forth alternativeembodiments, other embodiments of the present invention are alsocontemplated, as noted in the discussion. In all cases, this disclosurepresents illustrated embodiments of the present invention by way ofrepresentation and not limitation. Numerous other modifications andembodiments can be devised by those skilled in the art which fall withinthe scope and spirit of the principles of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments described as follows, address the long feltneed by those in the off road vehicle and motorcycle industries toprovide a reliable hydromechanical braking system which is responsive toprimary and secondary subsystem actuation forces using the same flowcircuits and simultaneously maintain independence of operation betweenthe two subsystems, such that failure of one of the two subsystems willnot disable or impair operation of the other subsystem. The presentinvention utilizes interrelated hydromechanical sensing, regulating andpressure boosting devices in a dual input operated vehicle brake systemto improve rider safety by eliminating potential brake loss in the eventof an unplanned brake line or caliper failure or disablement. Thepresent invention also reduces the necessity for providing brake systemmaintenance by eliminating routine mechanical brake adjustments.Further, the present invention provides enhanced brake systemflexibility through simplified adaptation to related but differentapplications. It will be apparent from the discussion herein below, thepresent invention provides completely independent operation of multipleinterrelated hydraulic brake subsystems while allowing these subsystemsto operate simultaneously in the same hydraulic circuit from a singlefluid feed reservoir. Finally, the present invention allows a dualmaster cylinder system to operate a brake caliper(s) independently fromeither the primary or secondary master cylinder(s), or simultaneouslyfrom both master cylinders without requiring the use of separate anddistinct caliper chambers, pistons or external mechanical actuatingdevices.

Looking now at FIG. 1, a simplified block diagram illustrating use of ahydromechanical fuze assembly 200, flow control valve assembly 400 andsecondary subsystem pressure booster (secondary master cylinderassembly) 300 to form a hydromechanically regulated vehicle brake system100 in accordance with one embodiment of the present invention is shown.The system 100 illustrated shows the hand master cylinder 108 adaptedfor use as the primary master cylinder and the foot master cylinder 300adapted for use as the secondary master cylinder. The present inventionis not so limited however, and it will readily be appreciated that thesystem 100 can also be easily adapted to use the foot master cylinder300 as the primary master cylinder for the system 100 and the handmaster cylinder 108 as the secondary master cylinder. With continuedreference to FIG. 1, operation of the vehicle brake system 100 will bedescribed herein below for a plurality of different operating conditionsto clarify overall system 100 operation and identify operationalinterrelationships among individual system 100 components.

The hydromechanical fuze assembly 200 senses hydraulic fluid flow rate,flow volume, flow direction, secondary system back pressure andtemperature during operation to prevent loss of vehicle brakes in theevent a secondary system is damaged to the point of serious hydraulicfluid leakage. The fuze assembly 200 provides for specific regulatedflow volume and seals off the secondary system if the flow volume isexceeded. The fuze assembly 200 generally senses system integrity,discriminating between system damage and unanticipated caliper piston"push back". Further, the fuze assembly 200 prevents system leak downwhen slow leaks occur during use and/or storage periods.

The present inventive in-line secondary master cylinder assembly 300provides for secondary system fluid flow and pressure, eitherindependently or simultaneously with the primary system. The secondarymaster cylinder assembly 300 in-line feature allows the entire unit tobe pressurized, eliminating any need for a vented fluid reservoir, orline running to a remote reservoir. The secondary master cylinderassembly 300 operates in a prepressurized system because it has nocompensation port, allowing the assembly 300 to generate pressureregardless of system starting pressure, without seal damage generallycaused by passing over a compensation port while under pressure. Asdiscussed further herein below, the assembly 300 is fully pressurerelieving at the end of its return stroke, ensuring system equilibriumis reached at the end of every stroke. Furthermore, the assembly 300 canbe mounted in any orientation with only air bleed port changes required,is capable of operating multiple downstream calipers and devices atremote locations where mechanical linkages would be difficult toutilize, and supports elimination of mechanical secondary brakingdevices which require periodic adjustments to remain functional.

The subsystem hydraulic input flow control valve assembly 400 is used inthe present invention to positively separate a primary system from asecondary system, while allowing both systems to function simultaneouslyin the same hydraulic circuit 100. The control valve assembly 400addresses the need for independent operation of the primary andsecondary systems while allowing both systems to share the samehydraulic circuit 100. Further, the assembly 400 effectively eliminatesthe necessity for dual lines to a single brake caliper to enable dualbrake system operation. As discussed further herein below, the presentinventive valve assembly 400 allows independent operation of a brakecaliper(s) from the primary and/or secondary system pressure sourceswithout having to use separate chambers or pistons in the caliper viadirecting input flow from the dominant flow and pressure source.

Stroking the primary master cylinder 108 input uniformly pressurizes theentire vehicle brake system 100 under normal operating conditions. Asthe vehicle operator continues stroking the primary master cylinder 108,normal system braking is achieved and the operator will observe anoticeable increase in the firmness associated with continued stroking.Continued stroking causes hydraulic fluid to flow through the hydraulicfluid lines 116, 117, 119 and through individual system 100 components200, 400, 300, 110, 112, 114, 120 until the desired pressure isachieved. The fuze assembly 200, described in detail herein below,operates in its normal regulated flow zone so long as the system 100integrity remains stable, and therefore has no effect on brake system100 operation. Under stable brake system 100 conditions, the hydraulicinput flow control valve assembly 400 is seated in its low position. Thesecondary master cylinder 300 remains unstroked and uniformlypressurized via the free backflow port from the primary system includingprimary master cylinder 108.

A steady state input via the primary master cylinder 108 results in auniformly pressurized system 100 wherein all system 100 components108-400 are uniformly pressurized. When a single input stroke isestablished as firm and stationary, the primary master cylinder 108 thenbecomes pressurized and stationary via a single input partial strokeunder normal operating conditions. In the absence of further strokingvia the primary master cylinder 108, the fuze assembly 200 slowlyreturns to its normal unstroked position away from the metering ballreturn spring 208, where it continues to have no operational effect onthe overall brake system 100 operation. Hydraulic input flow controlvalve assembly 400 remains seated in its low position. As before, thesecondary master cylinder 300 then continues to remain unstroked anduniformly pressurized via the primary system including the primarymaster cylinder 108.

All system 100 components 108-400 become uniformly depressurized when adecreasing pressure input results from discontinued primary mastercylinder 108 stroking. This condition normally occurs when a brake leveror handle, for example, is returning from a stroke during which thevehicle operator will notice a markedly softening feel associated withthe braking lever stroke. During the depressurization process, hydraulicfluid is returning to the primary master cylinder 108 via the systemhydraulic fluid lines 116, 117, 119. The backflow of system hydraulicfluid maintains the hydromechanical fuze assembly 200 in areturning-to-sealed or fluid free flow condition where it remains in asteady-state position ready to begin operation in its normal regulatedflow zone or alternatively ready to seal itself in the event to brakesystem 100 failure. Depressurization also has no effect on operation ofthe hydraulic input flow control valve assembly 400 or operation of thesecondary master cylinder 300 which remains transparent to the primarysystem unless it is actuated to close the free backflow port 348. If thesecondary master cylinder 300 generates flow and pressure exceeding theprimary system pressure, it will seal off the primary system through theinput flow control valve assembly 400.

Stroking the secondary master cylinder 300 via a foot brake pedal, forexample, until the stroking becomes firm, causes the secondary mastercylinder 300 to generate hydraulic fluid flow through hydraulic fluidline 116 into the rear brake caliper(s) 112. Only the rear brakecaliper(s) 112 can receive hydraulic fluid flow generated via thesecondary master cylinder 300. Stroking the secondary master cylinder300 has no operational impact on the primary master cylinder 108 whichremains unstroked and stationary. However, it will be appreciated thatsudden instantaneous stroking of the secondary master cylinder 300 willcause a small hydraulic fluid surge into a portion of the primaryhydraulic circuit 116, 117. The hydromechanical fuze assembly 200remains in its normally sealed position while the secondary mastercylinder 300 is stroking. The hydraulic input flow control valveassembly 400 changes its operational state when back pressure caused bystroking the secondary master cylinder 300 forces the input flow controlvalve 400 to switch from its normal seated low position to a seated highposition causing the primary system 116, 117 and related components 108,120, 200, 110, 114 to be isolated from the secondary brake system 119and rear caliper(s) 112. This condition wherein the secondary mastercylinder 300 is stroking and generating hydraulic fluid flow forces asplit piston, described herein below, within the secondary mastercylinder 300 to close. Flow volume is generated primarily via thedisplacement of fluid by the piston shaft 316 with a small flow volumedrawn from the primary brake system defined by that portion of thevehicle brake system 100 which is located on the opposite side of theinput flow control valve assembly 400 connected to the secondary mastercylinder 300.

A steady state secondary brake system input caused via an operatorpressing firmly down on a vehicle brake pedal, for example, willpressurize only the rear caliper(s) 112 and line 119, as stated hereinabove. If the primary master cylinder 108 remains unstroked,unpressurized and stationary, the hydromechanical fuze assembly 200 willthen remain sealed in its upper fuze seal position and the hydraulicinput flow control valve assembly 400 will remain seated high toeffectively isolate the primary master cylinder 108 from the secondarybrake system rear caliper(s) 112. The stroked position of the secondarymaster cylinder 300 causes the split piston portion of the secondarymaster cylinder 300 to close, thereby allowing the secondary mastercylinder 300 housing to generate pressure regardless of primary systemstatus.

Upon decreasing the secondary brake system pressure, e.g. removingpressure from the secondary brake pedal which causes a softening feel bythe operator on the return stroke of the brake pedal, the rearcaliper(s) is depressurized, allowing hydraulic fluid to return to thesecondary master cylinder 300. If the primary master cylinder 108remains unstroked, unpressurized and stationary, the primary mastercylinder 108 may receive hydraulic fluid from the secondary mastercylinder 300 while the secondary master cylinder 300 is beingdepressurized. During conditions of decreasing secondary brake systeminput and unpressurized, stationary primary brake system input, thehydromechanical fuze assembly 200 remains sealed on its upper seal oralternatively, in the free flow zone 224, and the hydraulic input flowcontrol valve assembly 400 remains seated high to isolate the primarymaster cylinder 108 and system 116, 117. Decreasing the secondary brakesystem input allows an internal piston spring, described herein below,within the secondary master cylinder 300 to return the split piston toits normal unpressurized position such that hydraulic fluid drawn fromthe primary portion of the brake system 100 is allowed to return to thatportion of the brake system 100.

It is to be appreciated that the inventive vehicle brake system 100 canalso be operated with simultaneously increasing primary and secondarybrake system inputs such that the primary master cylinder 108 and thesecondary master cylinder 300 as well as all brake system 100 components108-400 are uniformly pressurized. This condition is representative ofthe situation which occurs when the primary and secondary brake handles,levels, pedals, etc. (hereinafter levers) are both becoming firm withminimum stroke, thereby generating hydraulic fluid flow and pressurewithin the primary and secondary master cylinders 108, 300 as the brakestroking continues. During conditions of simultaneous primary andsecondary brake system stroking, the hydromechanical fuze assembly 200also strokes within its regulated flow zone while the hydraulic inputflow control valve assembly 400 floats with the dominant hydraulic fluidflow direction. The present invention can therefore provide differentialbraking to front versus rear brake calipers, for example, to change theratio of front to rear brake forces.

A condition of equal steady-state primary and secondary brake systeminputs, indicative of the primary and secondary brake levers being firmand stationary, results in a uniformly pressurized vehicle brake system100. This condition is capable of fully pressurizing the primary mastercylinder 108 with only a partial input stroke. The steady state inputsallow the hydromechanical fuze assembly 200 to slowly return to itsnormal unstroked position while allowing the hydraulic input flowcontrol valve assembly 400 to float, as stated above, with the dominantoil flow direction. The steady state inputs also maintain the secondarymaster cylinder 300 in a stroked and stationary condition such that theaforesaid internal split piston is forced to close and allow thesecondary master cylinder 300 housing to fully pressurize.

Yet another mode of operating the present inventive brake system 100occurs when the primary and secondary brake system inputs aresimultaneously decreasing causing all system components 108-400 touniformly depressurize. This condition can result, for example, whenboth (primary and secondary) brake levers are returning from an inputstroke and softening or releasing the input applied pressure. Asdescribed above, this condition also allows hydraulic fluid to return tothe primary master cylinder 108 from the pressurized system 100. As thesystem pressure decreases, the hydromechanical fuze assembly 200 returnsto its normal upper seal position and the hydraulic input flow controlvalve assembly 400 operates in its floating condition controlled by thedominant hydraulic fluid flow direction. The decreasing pressure alsoallows the secondary master cylinder 300 internal piston spring tooperate, forcing the split piston to return to its normal unpressurizedposition such that hydraulic fluid is returned to the primary portion ofthe vehicle brake system 100 as well as the displaced volume of thesecondary master cylinder 300 housing.

The present invention also accommodates the unique operational situationwhereby a primary brake system input overtakes a secondary system input,e.g. the secondary brake system pressure is overcome by a higher primarybrake system pressure. This unique situation may result, for example,when increasing the primary brake pressure provides total brake system100 control as the primary brake system input is being stroked andgenerating hydraulic fluid flow. During the takeover process, thehydromechanical fuze assembly 200 will continue to be stroked, but withreduced volume, while the hydraulic input flow control valve assembly400 will shift from its upper seal operating position to its lower sealoperating position. The aforesaid operational characteristics of thesystem components 200, 400 will allow the secondary master cylinder 300internal pressure to increase with increased primary system pressurewhile the secondary master cylinder 300 input pressure remainsunaffected.

In contradistinction to the aforesaid unique operational situation, thesecondary brake system input may also overtake the primary brake systeminput to control the rear caliper(s) 112 when the secondary brake systeminput pressure exceeds the primary brake system input pressure.Operationally, the system operator may sense a slight pulse in theprimary input lever as the hydraulic input flow control valve assembly400 shifts its operating mode. When the secondary brake system inputovertakes the primary brake system input, the hydromechanical fuzeassembly 200 will return to it upper seal operating position while thehydraulic input flow control valve assembly 400 shifts from its lowerseat position to its upper seat operation position leaving the secondarymaster cylinder 300 free to generate flow and pressure as it is stroked.

The present inventive brake system 100 can also operate effectivelywhile the primary and secondary inputs are oscillating such that thehydraulic pressure oscillates in all brake system calipers 110, 112,114. During the oscillation process, the hydromechanical fuze assembly200 and the hydraulic input flow control valve assembly 400 operatingmodes are also oscillating, i.e. reversing. The vehicle operator willsense pressure pulses in the brake inputs during the oscillation processwhich result from partial strokes generating pulsating pressures withinthe primary and secondary master cylinders 108, 300. The fuze assembly200 and flow control valve assembly 400 each have an internal flowcontrol ball mechanism, described herein below. The fuze assembly 200flow control ball mechanism flutters between its upper and lower sealoperating positions while the flow control valve assembly 400 ballmechanism flutters between its upper seat and its lower seat operatingpositions while the primary and secondary inputs are oscillating, toallow brake system control to reverse as desired by the vehicleoperator.

FIGS. 2A and 2B illustrate a detailed cutaway view of a conditionsensing hydromechanical fuze assembly 200 suitable for use in accordancewith one embodiment of the present inventive brake system 100 asdescribed herein above and as shown in FIG. 1. The condition sensingfuze assembly 200 is easily adaptable for use in any brake system whichmust maintain independence of operation between two or more subsystems,such that failure of one of the subsystems will not disable theother(s). The fuze assembly 200 can automatically sense brake systemconditions, resetting and adjusting itself to normal operation asdiscussed herein below.

The fuze assembly 200 safely enables primary and secondary hydrauliccircuit cross porting by ensuring that should the secondary portion ofthe hydraulic system fail, the primary portion will continue to operatenormally. Although the fuze assembly 200 is most preferably designed foruse with multiple manual input force vehicle brake systems, it willreadily be appreciated by those skilled in the art that other nonbrakeapplications will also benefit from use of the fuze assembly 200.Nonbrake systems which will benefit include but are not limited to thosesystems which require: 1) the ability to segregate one portion of thesystem from another based on regulated flow volume and rate; 2) theability to automatically reset based upon meeting a specific backpressure threshold; 3) operation with low back pressures, limitedbackflow volumes and exposure to temperature extremes without impairingthe fuze ability to operate and reset to the normal regulated operationmode; 4) availability of fluid viscosity compensation for applicationswhere fluid viscosity does not remain sufficiently constant to assureconstant fuze performance across a range of temperatures; and 5) normalsystem operation over very high and fluctuating pressure ranges.

The present inventive fuze assembly 200 will regulate operation of allsecondary subsystem components 112, 300, 400 from a single primarysubsystem input device 108 which may be hand and/or foot operated, forexample. When used in brake system 100 applications, where differentsize calipers or multiple calipers 110, 112, 114 are generally used, theregulated flow of the fuze assembly 200 is changed to ensure that onlythe correct volume of fluid is allowed to pass before setting. Once thefuze assembly 200 is set, it will automatically reset itself to normaloperation if the regulated portion 112, 116, 117, 300, 400 of the system100 has maintained integrity. This condition is sensed through theexistence of a small back pressure which can only be present if systemintegrity is preserved. The fuze assembly 200 does not require manualresetting unless a system failure has occurred and maintenance has beenperformed. In this case, upon completion of maintenance, a small amountof back pressure is introduced in the repaired subsystem to initiate thereset to normal regulated operation.

With continued reference to FIGS. 2A and 2B, the fuze assembly 200includes an upper body housing 202, a lower body housing 204, a meteringball 206, a return spring 208 and a retainer 210 as well as seals 212,214 and 216. The upper housing seal 214 provides a sealing function whenthe system 100 is static. It also serves as a stop to hold the meteringball 206 at position `A`, establishing the travel limit of one end ofthe regulated flow zone 220. The upper housing seal 214 further servesas a divider between the regulated flow zone 220 and the free flow zone224. The free flow zone 224 is provided to allow for system 100 airbleeding, backfilling and back flow relief. The regulated flow zone 220provides the minimum regulated flow volume necessary to reliably operatethe regulated subsystem devices 112, 300, 400. The regulated flow zone220 has a very precise metering ball 206 to nominal bore 230 clearancedimension to regulate fluid bypassing the metering ball 206. The presentinventors have found a clearance dimension ranging between 0.0001-inchand 0.0060-inch will provide working results for the present inventionwith various fuze body 202 and metering ball 206 material combinationsdescribed herein for further narrowing the range. A specific combinationof materials will determine the clearance necessary to achieve thedesired flow rate. A "best range" not greater than 0.0005-inch isestablished to achieve a desired flow rate for a specific set ofmaterial choices. The aforesaid rate of bypass has been found to be animportant parameter in establishing flow volumes to set the fuzeassembly 200 at position `B` or to return (reset) to position `A`. Abypass range of 0.0001 cu-in/sec to 0.09 cu-in/sec was found by thepresent inventors to be workable. A flow rate of about 0.009 cu-in/secfor the presently embodied geometry was found workable to accomplishapproximately a five second metering ball 206 return time. The rate ofhydraulic fluid bypass on the return to position `A` regulates theimportant fuze assembly 200 recovery time and flow volume rate forreturning to position `A`. In brake system applications, the bypass rateis a parameter of substantial interest, since it determines how quicklya temporarily disabled subsystem, which still possesses basic systemintegrity, will return to it normal operation as well as how fast theinput force must occur to set the fuze 200. When the metering ball 206comes to rest at position `B` of the checked zone 244, all hydraulicfluid flow through the fuze assembly 200 stops. At this position `B`,the metering ball 206 is spring 208 loaded against a retainer 210 with apredetermined preload. A preload range of about 1-8 ounce was found bythe present inventors to provide workable results for the presentinvention. The metering ball 206 will stay in this preload positionuntil a specific pressure differential between its two sides (upper bodyhousing 202 and lower body housing 204 ) is sensed, at which time itwill unseat and begin its regulated return to position `A`. A pressuredifferential of less than 7 psi was found preferable to unseat themetering ball 206 from its preload position `B`.

The present inventors have found that the present inventive fuzeassembly 200 can be used in locations where substantial climatic andenvironmental extremes may exist. Since fluid viscosity may varysubstantially over such temperature extremes, regulated operation wouldbe degraded if viscosity compensation could not be provided. Thiscompensation is provided by carefully selecting materials for the upperbody housing 202 and metering ball 206, such that the diametricalclearance between the bore 230 and the metering ball 206 will increaseor decrease at a rate which is approximately proportional to changesrequired to maintain relatively constant fluid flow rates around themetering ball 206 regardless of changes in fluid viscosity. Upper bodyhousing 202 materials comprising aluminum and steel and metering ball206 materials comprising aluminum, steel and polymers were found by thepresent inventors to provide optimal results. In cases where temperatureextremes are not encountered, lower cost materials of similar thermalcoefficients of expansion can be used without detrimental effects toregulated flows.

FIG. 5 is a graph illustrating the interactive effects of differentmaterials to precisely control hydromechanical fuze reset times inaccordance with one embodiment of the present invention. For example, athermal analysis of fuze assemblies 200 having identical bores 230 andball 206 sizes, but made from dissimilar materials, shows how materialselection provides precise tailoring of the fuze assembly 200 resettimes. A fuze assembly 200 comprising an aluminum body housing 202 and apolymer ball 206 has a nearly constant reset time when used attemperatures below approximately 120° Fahrenheit whereas a fuze assembly200 comprising various combinations of steel and aluminum displaysignificantly different reset characteristics over a temperature rangeof -40° F.-140° F. as shown. It can be seen that a specific hydraulicsystem can easily be tailored with a fuze assembly 200 which willprovide the desired results over a temperature range of interest.

As stated herein before, the upper body housing 202 comprises a freeflow zone 224. The free flow zone 224 provides for free flow of fluidaround the metering ball 206 which enables air bleeding back through thefuze assembly 200 and allows for vacuum and pressure filling of thebrake system 200, which are necessary in high volume productionmanufacturing. The upper body housing 202 also comprises a regulatedflow zone 220 for regulating fluid flow through the fuze assembly 200.Fuze assembly 200 performance within specified parameters is achieved bysetting the diametrical clearance between the metering ball 206 and thefuze assembly bore 230, setting the return spring force applied to themetering ball 206, and selecting body and ball materials which tend tocancel the effects of viscosity changes in the fluid over extremetemperature variations.

With continued reference to FIGS. 2A and 2B, the upper body housing seal214 prevents leakage of fluid through the assembly 200 when the meteringball 206 is stopped against it. This seal 214 also provides a stop forthe metering ball 206, locating the metering ball 206 at position `A`.Further, this seal 214 prevents the metering ball 206 from entering thefree flow zone 224 unless sufficient back pressure exists to push itpast the seal 214. This upper body housing seal 214 has been foundimportant to prevent system leakdown, even when integrity has been lostin the regulated subsystem, by blocking fluid flow through the fuzeassembly 200. Further, seal 214 prevents system 100 leakdowns duringextended storage by creating a slight vacuum in the regulated system112, 300, 400 as it gives up fluid.

The lower body housing 204 contains the checked flow zone 244 of thefuze assembly 200. When fluid flow parameters have been exceeded, themetering ball 206 enters the checked flow zone 244 where it is retainedby the retainer 210 unless there is sufficient differential pressure andflow to push it back past the retainer assembly 210 into the regulatedflow zone 220. The lower body housing 204 provides the metering ball 206seat which resists high pressures applied to the metering ball 206,holds the retainer 210 in its required position, holds the metering ballreturn spring 208 in its seat, and preferably threads into the upperbody housing 202 to allow assembly and disassembly of the fuze assembly200.

The retainer 210, as stated herein above, holds the metering ball 206 inthe checked flow zone 244 until sufficient differential pressure andfluid flow is exerted to urge the metering ball 206 back through theretainer 210. The present inventors have found the retainer 210 can bemade very sensitive by precise spring loading and design so thatrepeatable and reliable performance in response to specific thresholddifferential pressures can be achieved.

The booster seal 216 ensures against leakage around the metering ball206 when the metering ball 206 is retained in the checked flow zone 244.In systems where back pressures and flow are very slight, the boosterseal 216 provides the metering ball 206 with an additional boost to pushit past the retainer 210. In cold temperature applications, the boosterseal 216 contracts with temperature and provides an even larger boost toeject the metering ball 206 from the checked flow zone. Most preferably,the booster seal 216 is comprised of a material commonly known to thoseskilled in the art, to resist chemical attack in modern hydraulicbraking system applications.

The metering ball return spring 208 functions to urge the metering ball206 to position `A` where it is positioned normally to start eachstroke. The speed of metering ball 206 return is a parameter of greatinterest because such reset times determine how quickly a temporarilydisabled or over extended system will return to normal operation. Springforces are a factor in setting the regulated reset fluid flow rate. Inbrake system applications, this is a measure of how quickly a "pushedback" caliper piston condition will be returned to normal operation andalso how large a flow rate will be required to set the fuze assembly 200as well as the speed necessary to set the fuze assembly 200 to preventfluid flow around the metering ball 206.

FIGS. 3A and 3B illustrate a detailed cutaway view illustrating amanually operated in-line secondary master cylinder 300 suitable for usein accordance with one embodiment of the present invention as shown inFIG. 1. This manually operated in-line secondary master cylinder 300 canbe adapted for use with any hydraulic system which accommodates multiplemanual input forces, and which requires the secondary input device tooperate independently of the primary input device. The particularembodiment 300 is illustrated for use in a hydraulic brake system 100which must accommodate both hand and foot brake inputs to the brakesystem, either independently or simultaneously. The present inventivesecondary master cylinder 300 can be employed in any applicationwhere: 1) it is necessary or desirable to have more than one mastercylinder to generate fluid flow and pressure in order to operate one ormore subsystems independently of the primary master cylinder inputs, 2)an emergency or backup source of pressure is desirable without theencumbrances of maintaining two physically separated systems, 3) anin-line pressure booster is needed to step up pressure provided by aprimary system, 4) the need exists to initiate a pressure stroke in asystem which may already have been pressurized from a different source,5) a need exists to provide for completely independent operation of twointerrelated hydraulic pressure sources, but also allow theseindependent sources to operate simultaneously in the same hydrauliccircuit, and/or 6) a need exists to allow for the elimination of anyconnection to a remote reservoir, except through existing systempressure lines, although it will readily be appreciated by those skilledin the art, the present invention is not so limited.

Significantly, the present innovative secondary master cylinder 300 mayincorporate a fully pressurized body housing 318. This feature allowsthe secondary master cylinder 300 to be placed directly in a primarysystem operating pressure line 116, where it has no effect on systemoperation until it is actuated. Unlike a more typical master cylinder,the present secondary master cylinder 300 draws a very small amount ofcharging fluid directly from the primary system line 116. This action isaccomplished using a split piston, free backflow, volume displacementapproach, discussed herein below. The secondary master cylinder 300 isgenerally actuated by manual operator inputs.

The secondary master cylinder 300 can be easily adapted to generate flowinto a downstream line only simply by placing a check valve in the inputport 324. When the secondary master cylinder 300 is utilized as anindependent brake system actuator, such as in combination hand and footoperated vehicle brake system 100, it must be used with a flow controlvalve such as the hydraulic input flow control valve 400 designedspecifically for brake system applications and illustrated in FIGS. 3and 4. When configured as a secondary master cylinder in a brakepressure line application, the present inventive secondary mastercylinder 300 can be specially valved by an input flow control valvehaving a lengthened ball travel to provide a pressure boost to primarysystem brake pressure for increased brake system effectiveness bypersons who are handicapped or have inadequate strength to operate theprimary system without additional force input. Alternatively, thesecondary master cylinder 300 can also be reconfigured as the primarypressure source. This flexibility in application provides a substantialadvantage over standard vented reservoir master cylinders familiar tothose skilled in the art.

With continued reference to FIGS. 3A and 3B, it can be seen that as thepiston 302 is urged further into the bore 306 of its housing sleeve 304,its tapered face 308 and piston face seal 310 makes contact with themating tapered face 314 of a floating piston 312. The floating piston312 movement is restricted by clip 334 which is concentric with the bore320 of the secondary master cylinder housing 318. As the piston 302 andfloating piston 312 are stroked forward and urged closer together,hydraulic fluid is displaced through an exit port 322. The volume offluid displaced is equal to the volume reduction of the housing bore320. As the piston 302 commences its return stroke, it separates fromthe floating piston 312 when pressure drops to less than the amountnecessary to maintain the floating piston 312 against the tapered face308 of the piston 302, or when the floating piston 312 reaches thepositive stop face 326 of the piston sleeve 304. At this moment in time,pressures on both sides of the piston 302 are equalized and fluid isallowed to free flow through small grooves 344 between the tapered face314 of the floating piston 312 and the piston sleeve 304. This actionallows the pumping chamber (housing bore 320) to be recharged withhydraulic fluid and to equalize pressures within the housing 318 and theinput port 324.

The arrangement illustrated in FIGS. 3A and 3B provides several distinctadvantages for the present inventive brake system 100 over those brakesystems known to those skilled in the art. For example, the secondarymaster cylinder 300 pressure stroke can be initiated regardless ofexisting pressures in the housing 318 and output port 322. Unlikecurrent master cylinders, this arrangement does not require acompensation port in the pressure generating chamber. Most of thedisplaced hydraulic fluid is produced by volume reduction of the pumpingchamber 320, which is being filled with the piston 302 itself. Thus, forsingle stroke applications, very little system fluid is required.Further, the secondary master cylinder 300 can be mounted in anyorientation with only air bleed port location changes. Where redundantoperation of end use device(s) is desirable, the present secondarymaster cylinder 300 can eliminate the need for two separate flow linesto end use device(s), as well as the necessity for two chambers withinthe device(s). The secondary master cylinder 300 can also be readilyswitched over to a stand-alone configuration for use in otherapplications simply by connecting a return line to the input port 324and replacing the fluid chamber cap 328 with a removable vented standpipe type reservoir.

In summation, the secondary master cylinder housing 318 contains theworking components of the pumping assembly 300 and provides thenecessary fluid flow passages, connections and bores for the assembly300 to operate. The housing 318 is fully pressurized to offer fullpressure connection at the fluid input port 324 and fluid output port322. The housing 318 mounts in any orientation with no need for ventingor providing a separate reservoir, although air bleeding may still benecessary in specific applications.

The piston sleeve 304 houses and protects the piston 302 and completelyenvelopes the piston 302, providing substantial support to resistactuation side forces from eccentric end loading. The sleeve 304 alsoshields the piston 302 against extremely wet and dirty environments.

The piston 302 transmits the input force via the hydraulic fluid andcarries and engages the floating piston 312 such that sealing isobtained and consummated by the relative motion between the twocomponents 302, 312. A tapered face 308 provides a substantially largerbearing surface to reduce the surface contact stresses and potentialcomponent wear and to provide positive centering of the floating piston312. The tapered face 308 also provides a structure to retain the faceseal 310 and also provides a double seal (tapered metallic face 308 totapered metallic face 314 as well as via face seal 310). Further, aspherically shaped actuation cup 350 provides for use in applicationswhere the actuating input device(s) may rock through an arc of motion.

A floating piston 312 seals against the piston 302 and the bore of thehousing 318 to effect pumping and pressure generation as it floats backand forth on the piston 302 to open and close the flow passages 344 onits interior. As stated herein above, the floating piston 312 providespressure equalization when it engages the leading edge 326 of the pistonsleeve 304 and separates from the sealing surface 308 of the piston 302,preventing trapped pressure at the end of the return stroke.

A retainer 334 limits the travel of the floating piston 312, fixing themaximum clearance between the piston 302 and the floating piston 312tapered faces 308, 314 while a return spring 330 returns the piston 302to its initial unstroked position. Washers 332 are provided to providestructural integrity and secure the return spring 330. With continuedreference to FIG. 3A, it can be seen numerous additional seals 336, 338,340 are provided at strategic locations to prevent fluid leakage betweenmoving surfaces and a wiper 342 is additionally provided to maintainpiston 302 surface integrity and promote piston 302 longevity.

Moving now to FIG. 4, a detailed cutaway view illustrating a subsystemhydraulic input flow control valve assembly 400 suitable for use inaccordance with one embodiment of the present invention as shown in FIG.1 is shown. The input valve assembly 400 can be easily adapted for usein any vehicle brake system which accommodates multiple, e.g. hand andfoot, manual input forces and which must maintain independence ofoperation between the hand and foot hydraulic subsystems such thatfailure of one of the two subsystems will not disable or adverselyimpair the performance of the other. Although the input valve assembly400 comprises a plurality of specific features for use in hydraulicbrake systems, the assembly 400 can be easily adapted for use in otherapplications which require two hydraulic input flows to be directed to asingle output port such that: 1) the lower pressure supply will besealed off from the higher pressure supply when there are differentpressure inputs, 2) an intermediate position exists during which bothinputs can supply hydraulic fluid to the output simultaneously, 3) veryhigh reliability is inherent and essential in long term service due toserious consequences of failure, 4) the total flow area through thevalve assembly 400 must remain very high throughout the transition froma primary flow position 404 to a secondary flow position 406 to offerminimal flow restriction at all times during repositioning of the valveassembly 400, 5) very low pressures and low flow volumes can accomplishthe valve assembly 400 position 404, 406 transitions very quickly, 6)the ability to embody the valve assembly 400 within other system devicesto save space and cost is desirable, 7) movement of the valving ball 408does not result in perceptible back pressures in switching or returningfluid flows, and 8) the design approach must accommodate high volumeproduction and assembly rates for low cost applications, although itwill readily be recognized by those skilled in the art, the presentinvention is not so limited.

The present inventive hydraulic input flow control valve assembly 400has a flow geometry which provides very high flow volumes through thevalve assembly 400 at all times and which requires an extremely shortlength of travel for the valving ball 408 to travel from one seatposition 404 to the opposite seat position 406 as depicted in FIG. 6.The valve assembly 400 operates effectively with only a very smallvolume of hydraulic fluid required for a closed port to be opened to afull flow position. The unique flow valve assembly 400 structureutilizes a two piece centered half construction process which reducescost and improves quality by enabling each valve seat 404, 406 to beheld concentric only to its adjacent bore, and to machine one of thehalves directly into an adjoining component part 318, 402. This uniquestructure provides a method of obtaining a precise flow gap in thecenter of the valve assembly 400.

Looking again at FIG. 4, it can be seen that the input flow controlvalve assembly 400 has only one moving part, the checking ball 408 whichtravels a very short distance between its two seat positions 404, 406.As stated herein above, the valve assembly 400 is designed preferablyfor use in multiple manual input force brake systems. Specifically, itis the component which seals off the primary subsystem from thesecondary subsystem when hydraulic fluid flows are switching from one tothe other. The input flow control valve assembly 400 also allowssimultaneous operation of both subsystems as stated herein before. Ifthe primary subsystem becomes damaged or inoperative, the input flowcontrol valve assembly 400 automatically reacts to the lower pressureand seals the damaged or unused portion of the subsystem from theremaining subsystem to ensure continued operation of a portion of thevehicle braking system. The input flow control valve assembly 400 can behoused and mounted independently, or can be incorporated into otherdevise bodies as it is shown in FIG. 3A. The valve assembly 400 can beseen to utilize three-way fluid flow ball checking valve principles,comprising the ability to accommodate simultaneous flow in all threeports. The present inventors have therefore recognized numerous problemsassociated with dual input manual brake systems and have applied theaforesaid principles to arrive at the present novel solutions to thoseproblems.

With continued reference to FIG. 4, the flow control valve assembly 400comprises an upper body housing 402 which provides the upper seatposition 406 for the checking ball 408 to seal against and also providesa guiding bore 420 for the ball 408 to travel within as it approaches orleaves the upper seat position 406. The upper body housing 402 comprisesa novel structure which ensures the base of the upper body housing 402will stay a specific distance away from the mating half lower bodyhousing 318 to provide a precise gap for fluid flow without thenecessity for machining a special flow port. This large diametrical gap430 at the center of the valve assembly 400 provides a very large 360degree flow area for fluid to be directed out the exit port 322 of thevalve lower body housing 318, and facilitates the extremely short travellength of the ball 408, as well as reduced manufacturing costs. Thisaforesaid short travel length is very important in manual brake systemswhere there is very limited fluid available from the master cylinder tooperate the brakes.

The checking ball 408 is positioned to travel back and forth within thevalve assembly 400 to check the flow of fluid from the port having thelower pressure, e.g. port 324 or port 440.

The lower body housing 318, as stated above, is largely the same as theupper body housing 402 from a fluid flow standpoint, but additionallymust include provisions for removably mating with the threaded portionof the upper body housing 318 and provide a sealing surface for thestatic body seal necessary to keep the assembly 400 from leaking afterit is fully assembled. Simple O-Ring seals 410, 412 are provided toprevent leakage from occurring between the two body housing portions402, 318.

FIG. 6 is a graph illustrating the efficiency of the input flow controlvalve assembly 400 depicted in FIG. 4 based upon actual valve assemblyflow area as a percentage of maximum practical fluid transmission lineflow area for a standard 3/16-inch fluid line as the checking ball 408travels from its low seat position 404 to its upper seat position 406.It can be observed that as the checking ball 408 travels from it lowseat position 404 to its upper seat position 406 and its centerlinepasses the lower edge of the flow groove 430, the valve assembly 400flow area remains approximately constant. This is because any flow arealost on one side of the checking ball 408, as it passes into the flowgroove 430, is gained on the other side until the centerline of thechecking ball 408 passes the upper edge of the flow groove 430. Theembodiment characterized by the graph in FIG. 6 was found by the presentinventors to require a total analytical displacement volume of only0.009 cubic inch of fluid to move the checking ball 408 from its lowseat check position 404 to its upper seat check position 406, making theinventive control valve assembly 400 extremely sensitive and fast toreact to flow input changes. The present inventors found that it wasvirtually impossible in actual applications to balance the checking ball408 in suspension between the low seat position 404 and the upper seatposition 406 due to its extreme sensitivity to pressure differentials.Because the checking ball 408 is essentially friction free during itstravel periods, the amount of time during which the flow area is lessthan 100% is undetectable in actual use. The graph illustrated in FIG. 6was determined with a reference base flow area comprising 75% of a 3/16inch fluid transmission line as the full line diameter is not availablein actual practice due to system flow restrictions caused by fittings,connectors, porting and tolerances.

Keeping the foregoing detailed descriptions of the present inventivesystem and component structures, functions and operations in mind, aplurality of potential brake system problems and use of the presentinvention to cure those problems is discussed herein below. For example,a slow fluid leak can occur in the secondary brake system during use.When this condition occurs, the hydraulic fluid reservoir replenisheslost fluid as required and the secondary brake continues to functionnormally from both master cylinders 108, 300. The vehicle will noticethat as the primary reservoir continues to lose fluid, the primarymaster cylinder 108 will go soft, while the secondary master cylinder300 performs normally. The primary master cylinder 108 will not beaffected until the fluid level gets sufficiently low, at which time airenters the systems, causing the brake stroke to feel soft. The fuzeassembly 200 will continue to operate normally and will not set duringthis time period. Finally, reduced braking via the secondary mastercylinder 300 is felt by the operator as fluid continues to be lost.Thus, the present inventive hydraulic brake system 100 provides thevehicle operator with sufficient warning, allowing the operator tolocate and repair the slow fluid leak before complete brake systemfailure.

A slow fluid leak can also occur in the secondary brake system duringperiods of nonuse. When such a condition occurs, the fuze assembly 200will operate normally to seal itself and prevent further fluid loss. Theprimary brake system will then be unaffected as the fuze assembly 200prevents further system leakdown. If sufficient fluid remains in thereservoir prior to resumed use of the brake system, then use of thesecondary master cylinder assembly 300 may unset the fuze assembly 200,causing the inventive brake system 100 to operate as described aboveduring periods of use accompanied by a slow fluid leak.

A rapid fluid leak in the secondary system during use will leave thefront brakes 110, 114 unaffected as the fuze assembly 200 will set andseal off the secondary master cylinder 300 and associated caliper(s)112. The operator will notice the secondary master cylinder 300 inputsare soft and have no feel as the secondary brake will not be functionalfollowing setting of the fuze assembly 200. The operator will alsonotice the primary master cylinder 108 will feel soft on first use,until the fuze assembly 200 sets itself, at which time the primarymaster cylinder 108 input will feel normal to the operator. The rapidfluid leak in the secondary system during use will cause the fuzeassembly 200 to set upon the first use of the primary master cylinder108. Thus, the brake system ahead of the fuze assembly 200 will continueto function normally and the fuze assembly 200 will prevent fluid lossto the secondary system.

Performing system maintenance can sometimes introduce air into thesecondary system. When this condition occurs, the primary or secondaryinputs will be soft on first use. The vehicle operator will observe thatthe fuze assembly 200 has sealed off the secondary system, but thatapplying a secondary master cylinder 300 input will unset the fuzeassembly 200 when this condition has occurred. Following setting of thefuze assembly 200, the primary system will be isolated and stroking theprimary system will feel normal to the operator. If the fuze assembly200 has not been set following introduction of air into the secondarysystem, application of a primary system input may set the fuze assembly200 while subsequent application of a secondary system input will unsetthe fuze assembly 200. Air trapped within the secondary system can beremoved via brake system bleeding, a process familiar to those skilledin the art. During the bleeding process, the secondary master cylinder300 input will feel soft on first use. Continued use of the secondarymaster cylinder 300 will pull fluid from the primary system to supportthe bleeding process. Subsequent to bleeding, the secondary mastercylinder 300 will operate normally.

Maintenance procedures can also introduce air into the secondary mastercylinder assembly 300. A soft brake condition occurs as this air iscompressed in the secondary master cylinder assembly 300 due to pressuregenerated from the primary master cylinder 108. This softness may beunnoticeable until enough air enters the system to create sufficientlysoft brakes, at which time the fuze assembly 200 will set causing theprimary brakes to operate normally. Because the primary master cylinder108 can operate the entire brake system 100, the primary master cylinderinput will feel soft until sufficient air is bled from the system 100,or the fuze assembly 200 sets itself. The fuze assembly 200 will setfrom a primary system input and will unset from a secondary system inputif the secondary master cylinder 300 is filled with sufficient fluid, asstated herein before. Air which is discharged into a brake system linewill normally rise to the primary master cylinder 108 and pass throughthe fuze assembly 200, allowing the secondary master cylinder 300 to bebled.

A leak can occur in the primary system during use. Continual fluid lossin the primary system can ultimately cause the primary master cylinderreservoir to be depleted causing the primary master cylinder 108 to drawair and the input to feel soft. This condition will allow the fuzeassembly 200 to remain in its normal unset condition as the secondarybrake system functions normally using fluid from within its own systemand fluid from within the secondary master cylinder 300.

Similarly, a leak can also occur in the primary system during periods ofnonuse. This condition occurs, for example, when the primary mastercylinder 108 fluid is depleted through a slow drain down, but is notdetected until first use. The vehicle operator will notice a primaryinput which feels completely soft upon first use and a secondary inputwhich is functioning normally. The primary master cylinder 108 is thusdrained down and inoperative during which time the fuze assembly 200remains in its normal unset condition. The secondary master cylinder 300will continue to function normally, using fluid within its own internalsystem and also fluid which feeds down from the primary system fluidlines.

Another undesirable condition sometimes occurs when a rear brake caliper112 piston gets "pushed back" from its rotor. When this conditionoccurs, fluid is forced out of the caliper housing and back through thesystem, into the primary master cylinder 108 reservoir. Existing systemsmay lose all brakes on first use, but the present invention will use thefuze assembly 200 to seal off the rear system, providing metered amountsof fluid flow to reposition the rear caliper piston against the rotoruntil normal operation is restored without loss of the front brakes atany time. The operator will subsequently experience normal front brakeoperation and abnormal rear brake operation, i.e. the rear caliper(s)112 requires several braking cycles to return to normal operation. Thefluid which has been forced out of the caliper housing into the mastercylinder 108 is collected within the primary master cylinder reservoirand subsequently returned to the secondary system during the aforesaidseveral rear caliper(s) 112 braking cycles. During the rear caliper(s)112 braking cycles, the fuze assembly 200 cycles between its set andunset positions until system 100 operation returns to normal. Thesecondary master cylinder 300 is unaffected by "push back", and canreadily apply pressure to return the rear caliper(s) 112 to normaloperation.

The present inventive hydraulic brake system can be prepared for normalvehicle operation utilizing a "power fill" process during assembly. Thisprocess can be performed by evacuating the system from the primaryportion of the system and simultaneously filling the system with fluidvia the secondary portion of the system during which time all systemchecks remain open. The brake system 100 fills normally from thesecondary master cylinder 300, although the present invention is not solimited. Thus, the primary master cylinder 108 is evacuated and drawsfluid up from the secondary master cylinder 300 fill point, during whichtime the fuze assembly 200 also remains open to free flow. Uponcompletion of the foregoing "power fill" process, application of thefirst primary system input will put the fuze assembly 200 in its normaltravel range.

This invention has been described herein in considerable detail in orderto provide those skilled in the art with the information needed to applythe novel principles and to construct and use such specializedcomponents as are required. In view of the foregoing descriptions, itshould be apparent that the present invention represents a significantdeparture from the prior art in construction and operation. However,while a particular embodiment of the present invention has beendescribed herein in detail, it is to be understood that variousalterations, modifications and substitutions can be made therein withoutdeparting from the spirit and scope of the present invention, as definedin the claims which follow. For example, it will be apparent to thoseskilled in the vehicle brake system art that although particularcombinations of inventive hydraulic subsystems and subsystem componentshave been illustrated, that many other combinations of subsystems usingthe present inventive subsystem components will also work to provide theintended function of providing a vehicle brake system which accommodatesboth primary and independent secondary brake subsystem actuation forces,while maintaining independence between the two subsystems such thatfailure of one of the two subsystems will not disable the other, eventhough they share common hydraulic circuits.

We claim:
 1. A hydraulic brake system comprising:a primary mastercylinder; a primary hydraulic brake circuit coupled to said primarymaster cylinder; at least one secondary master cylinder; at least onesecondary hydraulic brake circuit wherein each said at least onesecondary master cylinder is coupled to a secondary hydraulic circuitselected from said at least one secondary hydraulic brake circuit; atleast one hydraulic input flow control valve assembly, wherein each saidat least one hydraulic input flow control valve assembly is coupledin-line with a secondary hydraulic brake circuit selected from said atleast one secondary hydraulic brake circuit such that each said at leastone in-line hydraulic input flow control valve assembly directs fluidinput flow to said selected at least one secondary hydraulic brakecircuit from a dominant flow source, wherein said dominant flow sourceis selected from any one of said primary master cylinder and said atleast one secondary master cylinder; and a hydraulic fuze assemblyhaving a first port hydraulically coupled to said primary mastercylinder via a first hydraulic coupling circuit and a second porthydraulically coupled to said at least one secondary master cylinder viaa second hydraulic coupling circuit, said fuze assembly adapted to sensehydraulic fluid flow volume passing through the fuze assembly betweenthe first port and the second port and isolate said at least onesecondary master cylinder from said primary master cylinder and saidfirst hydraulic brake circuit when a predetermined flow volume isexceeded within said fuze assembly and reset with a specificbackpressure.
 2. The hydraulic brake system of claim 1 wherein saidhydraulic fuze assembly is further adapted to sense at least one ofhydraulic fluid flow rate, hydraulic fluid flow direction, secondaryhydraulic circuit back pressure, and hydraulic fluid temperature.
 3. Thehydraulic brake system of claim 2 wherein said primary hydraulic brakecircuit comprises at least one brake caliper.
 4. The hydraulic brakesystem of claim 3 wherein said at least one secondary hydraulic brakecircuit comprises at least one brake caliper.
 5. The hydraulic brakesystem of claim 3 wherein said at least one primary hydraulic brakecircuit caliper comprises at least one front caliper.
 6. The hydraulicbrake system of claim 4 wherein said at least one secondary hydraulicbrake circuit caliper comprises at least one rear caliper.
 7. Ahydraulic brake system comprising:a primary master cylinder; a primaryhydraulic brake circuit coupled to said primary master cylinder; atleast one secondary master cylinder; at least one secondary hydraulicbrake circuit coupled to each said at least one secondary mastercylinder; and at least one of a hydraulic fuze assembly and a hydraulicinput flow control valve assembly, said hydraulic fuze assembly having afirst port hydraulically coupled to said primary master cylinder via afirst hydraulic coupling circuit and a second port hydraulically coupledto said at least one secondary master cylinder via a second hydrauliccoupling circuit, said fuze assembly adapted to sense hydraulic fluidflow volume passing through said fuze assembly and isolate said at leastone secondary master cylinder from said primary master cylinder and saidprimary hydraulic brake circuit when a predetermined flow volume isexceeded within said fuze assembly, said control valve assemblycomprising an in-line hydraulic input flow control valve coupled in-linewith said second hydraulic coupling circuit such that said control valvedirects fluid input flow to said at least one secondary hydraulic brakecircuit from a dominant flow source, wherein said dominant flow sourceis selected from any one of said primary master cylinder and said atleast one secondary master cylinder.
 8. The hydraulic brake system ofclaim 7 wherein said hydraulic fuze assembly is further adapted to senseat least one of hydraulic fluid flow rate, hydraulic fluid flowdirection, hydraulic back pressure within said second hydraulic brakecircuit, and hydraulic fluid temperature.